How often should I save to Core Data?

Question

I'm working on an application backed by Core Data. Right now, I'm saving the Object Context as and when I add or delete an entity to and from the Context. I'm afraid it will affect the performance, so I was thinking of delaying the save. In fact, I could delay it all the way until the application is gonna terminate. Is it too risky to save the data only when the application is about to close? How often should I call the save on Object Context?

I was thinking of having a separate thread handle the save: it will wait on a semaphore. Every time any part of the application calls a helper/util method to save the Core Data, it will decrement the semaphore. When it is down to zero, the "save thread" will do a save once and it increments the semaphore to a, say, 5, and then sleep again.

Any good recommendation? Thanks!

Answer 1

You should save frequently. The actual performance of the save operation has a lot to do with which persistent store type you're using. Since binary and XML stores are atomic, they need to be completely rewritten to disk on every save. As your object graph grows, this can really slow down your application. The SQLite store, on the other hand, is much easier to write to incrementally. So, while there will be some stuff that gets written above and beyond the objects you're saving, the overhead is much lower than with the atomic store types. Saves affecting only a few objects will always be fast, regardless of overall object graph size.

That said, if you're importing data in a loop, say, I would wait until the end of the complete operation to save rather than saving on each iteration. Your primary goal should be to prevent data loss. (I have found that users don't care for that very much!) Performance should be a close second. You may have to do some work to balance the frequency of saving against performance, but the solution you outline above seems like overkill unless you've identified a specific and significant performance issue.